<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
		<style>
			#outer{
				width:500px;
				height:500px;
				margin:0 auto;
				border:2px solid red;
			}
			#outer .inner{
				width:200px;
				height:200px;
				border:1px solid blue;
				/*外边距可以取负值*/
				margin-left: -2px;
				background:white;
				/*内边距不能取负值*/
				/*padding-left: -50px;*/
				
				/*外边距取负值,常用来做压边框效果*/
			}
		</style>
	</head>
	<body>
		<div id="outer">
			<div class="inner">123</div>
		</div>
	</body>
</html>
